1. Field of the Invention
An aspect of the present invention relates to an optical pickup, and more particularly, to an optical pickup which can prevent deterioration of a tracking error signal caused by an adjacent layer during the recording and/or reproduction of a predetermined data with respect to a multilayer recording medium having a plurality of recording layers.
2. Description of the Related Art
Recently, many studies have been performed to increase the information storage capacity of an optical disk. A recording medium having a multilayer structure in which a plurality of layers are provided has been suggested as a method to increase the information storage capacity of an optical disk. For example, a DVD format in which two recording layers are spaced with an interval of about 55 μm has been adopted as a standard. A Blu-ray disk, one of next generation recording media, has a double layer structure, each layer having about 25 GB storage capacity, and uses a blue-violet laser diode and an objective lens having a 0.85 NA (numerical aperture). The multilayer recording medium with two or more recording layers can increase storage capacity in proportion to the number of recording layers.
A differential push-pull (DPP) method has been widely known as one of several tracking methods for an optical disk. The DPP method is widely adopted in an optical disk system because the method can remove an offset of a push-pull signal due to the shift of an objective lens. In the DPP method, a beam is separated into three beams of the 0th order beam (a main beam) and ±1st order beams (sub-beams) using a grating. Next, the main beam and the two sub-beams are emitted onto the optical disk and a photodetector detects a reflection signal according to the emitted beams. The detected signal is output as a tracking error signal (TES) through a predetermined calculation process.
FIG. 1 is a view schematically illustrating optical paths during the reproduction of an optical disk having a multilayer structure according to a conventional technology. An effect by a reflection beam by an adjacent recording layer in a multilayer recording medium on the TES will be described with reference to FIG. 1. In FIG. 1, L0 and L1 denote recording layers of the multilayer recording medium.
Referring to FIG. 1, a beam emitted from a light source (not shown) passes through a diffraction grating (not shown) and is separated into a main beam M1 and two sub-beams S1 and S2. Then, the separated beams pass through a collimator lens 4 and an objective lens 1 and are emitted to the recording layer L1.
FIG. 2 is an enlarged view of a portion A of FIG. 1 and shows that the main beam M1 and two sub-beams S1 and S2 are emitted to the recording layer L1. Referring to FIG. 2, the main beam M1 is emitted to a track center TC formed on the recording layer L1 and the sub-beams S1 and S2 are emitted between a corresponding track and adjacent tracks. The main beam M1 and the two sub-beams S1 and S2 are reflected from the recording layer L1 and received by a photodetector 5. Also, a partial beam M2 of the main beam M1 is reflected from the recording layer L0, not from the recording layer L1 that is being reproduced, and received by the photodetector 5.
FIG. 3 is an enlarged view of a portion B of FIG. 1 and shows beam spots formed by the main beam M1 and the two sub-beams S1 and S2 detected by the photodetector 5. FIG. 4 is a view schematically illustrating the distribution of light detected by the photodetector 5 during the reproduction of the recording layer L1. Referring to FIG. 3, the photodetector 5 includes a main photodetector (MPD) receiving a main spot 10 of the main beam M1 and sub-photodetectors SPD1 and SPD2 receiving sub-spots 11 and 12 of the sub-beams S1 and S2, respectively. The main photodetector MPD is horizontally and vertically divided into four sections. Each of the sub-photodetectors SPD1 and SPD2 is divided into two sections. Assuming that the output signals of the respective sections of the photodetectors are A, B, C, D, E, F, G, and H, the TES is generated by calculating these signals. The TESDPP that is detected in the DPP method is calculated by an equation that TESDPP=[(A+B)−(C+D)]−k[(E−F)+(G−H)].
Referring to FIG. 4, a beam spot 20 of the partial beam M2 of the main beam M1 that is reflected from the recording layer L0 and received by the photodetector 5 is larger than the main spot 10 reflected from the recording layer L1 that is actually reproduced. Thus, the beam spot 20 reflected from the recording layer L0 may cover the sub-spots 11 and 12. Considering that the ratio of the light amount between the main beam and the sub-beam is typically set to 10:1, when the beam spot 20 formed by the partial beam M2 of the main beam M1 overlaps at least a part of the sub-spots 11 and 12, the beam spot 20 may affect a push-pull signal using the sub-spots 11 and 12. The beam spot 20 formed by the partial beam M2 reflected from the recording layer L0 affects the symmetry between the sub-spots 11 and 12 so that a DC offset may be generated in the push-pull signal.
As described above, in the multilayer recording medium, when part of a beam reflected from a recording layer, different from a recording layer that is to be reproduced, is detected by a sub-photodetector, the partial beam may greatly affect a push-pull signal by a sub-beam.
Referring back to FIG. 1, to prevent the deterioration of a tracking signal caused by a beam reflected from an adjacent layer, it has been suggested to include an optical member 2 for diffracting part of the light reflected from the adjacent layer and a ¼ wavelength plate 3 for changing polarization of incident light. According to the above arrangement, since the light reflected from the adjacent layer is diffracted by using the optical member 2, tracking servo crosstalk due to the adjacent layer can be reduced. The above arrangement may weaken the influence of a reflection signal caused by the adjacent layer without much loss of an RF signal when the distance between the recording layers is relatively great as in the double layer recording medium.
However, since the distance between the recording layers decreases as the number of recording layers increases, in order to reduce the influence of the reflection signal caused by the adjacent layer, the size of the optical member 2 needs to be increased. The increased size of the optical member 2 accordingly increases the amount of light that is diffracted by the optical member 2 so that the loss of the beam received by the main photodetector may increase and a jitter characteristic may be deteriorated.
As described above, in the multilayer recording medium, when the beam reflected by the adjacent layer, other than the recording layer that is being recorded or reproduced, is received by the photodetector, in particular, when part of the main beam having a relatively greater light amount is reflected from the adjacent layer and affects the sub-photodetectors, the push-pull signal created by the sub-beam is difficult to determine and an offset may be generated in the push-pull signal. As a result, tracking by a tracking servo error signal becomes unstable and the recording and/or reproduction characteristics may be deteriorated.